PROJECT SUMMARY G protein coupled receptor (GPCR) signaling pathways mediate actions of hormones and neurotransmitters. They are essential for normal function of the nervous system, frequently disrupted in many neuropsychiatric and neurological conditions and/or exploited for therapeutic purposes. While we learned considerable information about molecular players involved in traditional GPCR signaling, many critical gaps remain. Among biggest uncharted territories in the field is an issue of ?orphan? GPCRs, receptors with unknown signaling mechanisms. It is generally recognized that orphan receptors have tremendous potential for uncovering novel biology of the nervous system and harnessing it for potential therapeutic benefits. Our long- term goal is to understand principles in organization and functional regulation of poorly explored GPCR pathways in the effort to develop better treatments for brain disorders. The focus of our attention is on poorly understood orphan receptor- GPR158, yet one of the most abundant GPCRs in the brain. We found that GPR158 plays a pivotal role in stress-induced depression. Its expression is induced by glucocorticoids and is upregulated in patients with major depressive disorder. Conversely, GPR158 elimination in mice produces marked anti-depressant phenotype and stress resilience. At the molecular level, GPR158 recruits negative regulator of G protein signaling, RGS7 to the plasma membrane impacting production of the second messenger cAMP to control synaptic transmission and neuronal excitability. The proposal is focused on filling two biggest gaps in our understanding of GPR158 biology: its activation mechanisms and identity of effectors mediating its effects on neuronal circuitry underlying affective states. Intriguingly, our Preliminary Data revealed network of GPR158 extracellular interactions with the cell- adhesion like proteins and pointed to a cAMP-modulated K+ channel complex as a possible signaling mediator. Based on accumulated preliminary data we hypothesize that GPR158 transduces signals triggered by extracellular binding partners into changes of neuronal excitability by engaging inhibitory ion channel complex. This hypothesis will be tested by pursuing three complementary Specific Aims that seek to: (1) determine molecular mechanisms by which GPR158 transduces its signals, (2) dissect circuits that rely on GPR158 action to exert behavioral effects and (3) probe the involvement of the K+ channel complex in mediating the effect of GPR158 on neuronal excitability. The strategy proposed to address these Aims will entail a synergistic combination of biochemical, electrophysiological and cell-biological approaches, exploiting the existence of a powerful array of technologies and animal models. We hope that accomplishment of these goals will provide critical new insights into the mood regulation in mammals and suggest novel targets for the development of therapeutic interventions.